Check valve is a basic element of fluid control engineering and is applied very widely, playing a role of protecting other fluid machines and pipelines by stopping back flow of fluid. In the traditional check valve, a valve plate closes very fast, and the check valve generally mounted on the pipeline connected with the outlet end of a pump and closes to the outlet end of the pump for preventing back flow of fluid in the pipeline from damaging the pump, thereby protecting the pump. Currently, swing check valve and lifting check valve are commonly used in pipeline systems. Water hammer occurs in the pipeline system when a downstream valve is closed suddenly, and the check valve automatically closes due to flow break in the pipeline, so water hammer waves directly act on the check valve, and will damage the valve in severe case. The failure of a check valve is usually connected to water hammer accidents. Fast opening and closing of the check valve will cause water hammer which will damage the check valve, resulting in chain reaction in a loop and damage of more fluid parts (including other check valves) and thus making accidents more serious. The vicious circle will lead to serious consequence whichever link it starts.
The traditional check valves, no matter swing check valve, lifting check valve or inclined-disc type check valve, have a common characteristic that a valve plate is always in acceleration process during opening and closing such that moving parts of the check valve move too fast before stop, that is the direct cause of water hammer and valve clack impact of the check valve. In order to solve the problems of water hammer and valve clack impact of the check valve, three check valves which have dampers are developed: mechanical damped check valve, hydraulic damped check valve and hybrid damped check valve. The mechanical damped check valve has the advantage that the damping characteristic of the damper is easy to set through mechanical structures and the disadvantage that with the increase of flow velocity and flow quantity of fluid in a pipeline, the relative damping mechanism becomes huge and complex, and with the widening of variation domain of flow velocity and flow quantity in the pipeline, the opening-closing characteristics of the check valve at low flow velocity and quantity are deteriorated seriously (the check valve can not open and close normally in the condition of low flow velocity and quantity). The hydraulic damped check valve employs a special structure, such that the moving parts of the valve must do work to some part of fluid during opening and closing so as to reduce the kinetic energy of the moving parts, achieving the object of prolonging closure time and relieving valve clack impact. The scheme has the advantage that the check valve has simple structure and high operation reliability and disadvantages that the movement of fluid in the valve body is complex, the hydraulic damping characteristics are difficult to meet requirements through parametric design, design cost is high, and because damping of the hydraulic damper is increased with the increase of flow velocity and flow quantity in the pipeline, the hydraulic damped check valve has certain defects for high-flowrate pipelines having lower limit request on pipeline size. The hybrid damped check valve is a design combining the mechanical damped check valve and the hydraulic damped check valve, which optimizes the opening-closing characteristics of the check valve through complementary advantages. According to patens at home and abroad, this scheme have failed to make a breakthrough. A common shortcoming of damped check valves is that slow closing of the valve plate is liable to cause back flow of a part of water into a water pump, resulting in reverse rotation of the water pump.
In addition to the above check valves, there are silent check valve, direct impact check valve, water-hammer proof spherical check valve, etc. The silent check has low noise of closing because of short moving distance of the valve plate but can not avoid water hammer; actually, the direct impact check valve belongs to hydraulic damped check valves; and the spherical check valve has favorable water-hammer proof effect, but has high manufacture cost due to excessive complex structure and more sealing surfaces.